The present invention relates generally to well servicing operations, and, more particularly, to devices, systems and methods useful in stimulation blending for fluids, mixtures, and/or slurries used in well servicing operations.
Conventional blenders have been either the open top tub blenders, as shown in FIG. 1, or the centrifugal blender, as shown in FIGS. 2 and 3, such as are used on the Crown blenders or the programmable optimum density (POD) blenders. FIGS. 1a and 1b schematically illustrates a conventional blender 100 with an open top blending tub system 180. Fluids are introduced through an inlet 105, drawn in by a suction centrifugal 110, and then sent through an outlet 115 to a tub level valve 130 of the open top blending tub system 180. FIG. 1b schematically illustrates the open top blending tub system 180 of the conventional blender 100 shown in FIG. 1a. A pressure sensor 112 attached to the outlet 115, as indicated at 125, senses the pressure present in the outlet 115. The pressure sensor 112 sends the sensed pressure information to a pressure controller 114. The pressure controller 114 compares the sensed pressure to a pressure setpoint, as indicated at 114a, and sends pressure error control information to an hydraulic control head 116. The hydraulic control head 116 sends hydraulic control information to an hydraulic pump 118. The hydraulic pump 118 sends hydraulic fluid to an hydraulic motor 120. The hydraulic motor 120 drives the suction centrifugal 110, based on the pressure sensed by the pressure sensor 112, as controlled by the pressure controller 114 and/or the hydraulic control head 116.
As shown in FIGS. 1a and 1b, the tub level valve 130 receives the inlet fluid from the outlet 115 of the suction centrifugal 110 and sends the fluid to an open top tub 140, as indicated at 135. A level sensor 142 senses the level of the fluid and/or fluid/proppant mixture in the open top tub 140. The level sensor 142 sends the sensed level information to a level controller 144. The level controller 144 compares the sensed level to a level setpoint, as indicated at 144a, and sends the level controller output as a position setpoint to a position controller 136. The position controller 136 compares the position setpoint with the position of an actuator 132 from a position sensor 134 and sends position control information to a proportional valve 138. If the position error is negative, the proportional valve 138 will divert hydraulic fluid through a line 138a to the actuator 132 that is connected to and rotates the tub level valve 130. This rotation will increase the opening of the tub level valve 130. If the position error is positive, the proportional valve 138 will divert hydraulic fluid through a line 138b to the actuator 132 that is connected to and rotates the tub level valve 130. This rotation will decrease the opening of the tub level valve 130.
Proppant is introduced into the tub 140 through a proppant auger 140a, as indicated at 117. The speed of the proppant auger 140a is sensed by a speed sensor 140b. The speed sensor 140b sends the sensed speed information to a speed controller 140f. The speed controller 140f compares the sensed speed to a speed setpoint from a speed setpoint calculator 140g. The speed setpoint calculator 140g receives flow information from a flowmeter 115a (FIG. 1a) and also information from a proppant concentration setpoint, as indicated at 140h to calculate the speed setpoint sent to the speed controller 140f, as indicated at 115c. The speed controller 140f calculates the error between the speed setpoint from the speed setpoint calculator 140g and the speed sensor 140b. From the error, the speed controller 140f sends speed control information to an hydraulic control head 140e. The hydraulic control head 140e sends hydraulic control information to an hydraulic pump 140d. The hydraulic pump 140d sends hydraulic fluid to an hydraulic motor 140c. The hydraulic motor 140c drives the proppant auger 140a based on the speed calculated from speed setpoint calculator 140g. 
An agitation controller 146 receives input information from the proppant setpoint, as indicated at 140h and 119, and a discharge flowmeter 165a (FIG. 1a and 1b), as indicated at 165b. The agitation controller 146 calculates the required agitation and sends speed control information to a proportional valve 148. The proportional valve 148 sends hydraulic control information to an hydraulic pump 150. The hydraulic pump 150 sends hydraulic fluid to an hydraulic motor 152. The hydraulic motor 152 drives an agitator 154. The agitator 154 agitates the open top tub 140, mixing the proppant introduced through the proppant auger 140a with the fluid flowing into the open top tub 140 through the tub level valve 130, as indicated at 135. The resulting blend of fluid and proppant flows out of the open top tub 140 through an outlet 155 into a discharge centrifugal pump 160 (FIGS. 1a and 1b). The resulting blend of fluid and proppant flows out of the discharge centrifugal pump 160 to the downhole pumps (not shown) through the discharge flowmeter 165a and an outlet 165.
A pressure sensor 162 senses the pressure present in the outlet 165, as indicated at 175. The pressure sensor 162 sends the sensed pressure information to a pressure controller 164. The pressure controller 164 compares the sensed pressure to a pressure setpoint, as indicated at 164a, and sends pressure error control information to an hydraulic control head 166. The hydraulic control head 166 sends hydraulic control information to an hydraulic pump 168. The hydraulic pump 168 sends hydraulic fluid to an hydraulic motor 170. The hydraulic motor 170 drives the discharge centrifugal pump 160, based on the pressure sensed by the pressure sensor 162, as controlled by the pressure controller 164.
The open top blending tub system 180 must have a very robust tub level system to prevent either overflowing the open top tub 140 or running the open top tub 140 dry during normal operation. At the same time, the tub level must maintain a relatively constant inlet flowrate as measured by the flowmeter 115a to keep a steady proppant concentration. The proppant rate is proportional to the inlet flowrate, as determined by the tub level valve 130. However, good tub level control and constant inlet flowrate are contradictory requirements. As such, constant inlet flowrate must be compromised to prevent either running the open top tub 140 dry or overflowing the open top tub 140.
Changes in tub level also cause changes in the time constant for the open top tub 140 that, in turn, cause the proppant concentration to vary. Unless the volumetric responses of both the tub level valve 130 and the proppant auger 140a are exactly the same, the inlet proppant concentration will always be changing whenever the inlet flowrate is changing. Variations in tub level also cause the suction pressure to change to the discharge centrifugal pump 160. If the suction pressure to the discharge centrifugal pump 160 is too low, the discharge centrifugal pump 160 will lose prime and the downhole pumps (not shown) will cavitate. Furthermore, if the agitation is too high in the open top tub 140, then too much air will be beat into the fluid, thereby causing a reduction in the boost pressure and possible loss of prime of the discharge centrifugal pump 160. However, too low an agitation rate causes erratic proppant concentrations due to proppant falling out of suspension. In addition to the variations in proppant concentration, unless the tub level valve 130 and the liquid and dry additives (not shown) have the same time response, there will also be variations in the liquid and dry additive concentrations due to the changes in inlet rate to the open top tub 140.
The inlet rate to the open top blending tub system 180 will also vary due to the changes in the pressure in the suction centrifugal 110 on the conventional blender 100. There are many different potential failure modes in the conventional blender 100 with the open top blending tub system 180 that are primarily due to problems in the open top blending tub system 180.
FIGS. 2 and 3 schematically illustrate a conventional blender 200 with a centrifugal mixing system 260. Fluids are introduced through an inlet 205, drawn in by a suction centrifugal 210, and then sent through an outlet 215 to a mix/discharge centrifugal system 260. The mix/discharge centrifugal system 260 receives proppant, such as sand, from a proppant supply 270, and mixes the proppant received from the proppant supply 270 with the fluids sent through the outlet 215 from the suction centrifugal 210.
As shown in more detail in FIG. 3, a pressure sensor 312 attached to the outlet 215, as indicated at 325, senses the pressure present in the outlet 215. The pressure sensor 312 sends the sensed pressure information to a pressure controller 314. The pressure controller 314 compares the sensed pressure to a pressure setpoint, as indicated at 314a, and sends pressure error control information to an hydraulic control head 316. The hydraulic control head 316 sends hydraulic control information to an hydraulic pump 318. The hydraulic pump 318 sends hydraulic fluid to an hydraulic motor 320. The hydraulic motor 320 drives the suction centrifugal 210, based on the pressure sensed by the pressure sensor 312, as controlled by the pressure controller 314 and/or the hydraulic control head 316.
Similarly, a pressure sensor 362 attached to the outlet 265, as indicated at 375, senses the pressure present in the outlet 265. The pressure sensor 362 sends the sensed pressure information to a pressure controller 364. The pressure controller 364 compares the sensed pressure to a pressure setpoint, as indicated at 364a, and sends pressure error control information to an hydraulic control head 366. The hydraulic control head 366 sends hydraulic control information to an hydraulic pump 368. The hydraulic pump 368 sends hydraulic fluid to an hydraulic motor 370. The hydraulic motor 370 drives the mix/discharge centrifugal system 260, based on the pressure sensed by the pressure sensor 362, as controlled by the pressure controller 364 and/or the hydraulic control head 366. The proppant may be introduced to the mix/discharge centrifugal system 260 through an inlet, as indicated at 385.
The conventional blender 200 with the mix/discharge centrifugal system 260 has at least four major problems. The first problem results when the mix/discharge centrifugal system 260 is shut down prior to the suction system. When this happens, the mix/discharge centrifugal system 260 no longer acts as a centrifugal check valve and the suction fluid can be blown out the proppant inlet 270 which may result in a major environmental spill. If oil-based fluids are being pumped, a potential fire hazard may also result. The second problem results from larger quantities of volatile vapors being emitted due to pressures potentially lower than atmospheric pressure at the proppant inlet 270 and/or 385.
The third problem results from using the same device, the mix/discharge centrifugal system 260, both to mix and to boost the downhole pumps (not shown). Suppose only 15 pounds per square inch (psi) were used for mixing as opposed to 60 psi for mixing and providing boost to the downhole pumps. According to the affinity laws for centrifugal pumps, well known to those skilled in the art, the impeller speed must be twice as fast at 60 psi as compared to 15 psi.
By the same affinity laws, the wear rate in the centrifugal would be a cubic function of the ratio of the impeller speeds. This means that the wear rate in the mix/discharge centrifugal system 260 operating at 60 psi would be 8 times as great as a mixer system operating at 15 psi, since the impeller speed at 60 psi is twice that at 15 psi and the wear rate is then 23 =8 times as great. The fourth problem is the fact that this type of mix/discharge centrifugal system 260 consumes excessive horsepower, as described above with respect to the wear rate, and is, consequently, very inefficient. A good mixer is an inefficient pump and a good pump is an inefficient mixer. Since the same device, the mix/discharge centrifugal system 260, is used both to mix and to pump, overall efficiency is severely compromised.
U.S. Pat. No. 4,453,829 to Althouse, III, U.S. Pat. No. 4,614,435 to McIntire, and U.S. Pat. No. 4,671,665 to McIntire, show a conventional programmable optimum density (POD) mix/discharge centrifugal system that had problems due to also using this same programmable optimum density (POD) mix/discharge centrifugal system for a suction centrifugal. If any of the suction connections and/or hoses leaked air, then the suction side of this programmable optimum density (POD) mix/discharge centrifugal system would lose prime and the programmable optimum density (POD) mix/discharge centrifugal system would pack off with proppant and quit pumping.
U.S. Pat. No. 4,808,004 to McIntire et al., shows an improved conventional programmable optimum density (POD) mix/discharge centrifugal system that used a separate suction centrifugal pump to overcome the problems associated with using the same programmable optimum density (POD) mix/discharge centrifugal system for a suction centrifugal as well as for a mixing and a discharging centrifugal. The conventional blender 200 with the mix/discharge centrifugal system 260, as described above, similarly has a separate suction centrifugal 210.
U.S. Pat. No. 4,239,396 to Arribau et al., U.S. Pat. No. 4,460,276 to Arribau et al., U.S. Pat. No. 4,850,702 to Arribau et al., U.S. Pat. No. 4,915,505 to Arribau et al., and U.S. Pat. No. 6,193,402 to Grimland et al., show a similarly improved centrifugal mix/discharge system that used a separate suction centrifugal pump to overcome the problems associated with using the same mix/discharge centrifugal system for a suction centrifugal as well as for a mixing and a discharging centrifugal. In these systems, the discharge pressure is controlled by the suction pressure. These mix/discharge centrifugal systems provide a means for mixing the proppant and providing at least 5 psi boost above the suction pressure, so that there is a compromise between being an efficient pump and an efficient mixer. If the mix/discharge centrifugal system is shut down and/or goes down due to a failure prior to shutting down the suction centrifugal pump, then a geyser of fluid is sent out the proppant inlet of the mix/discharge centrifugal system.
The mix/discharge centrifugal system described in U.S. Pat. No. 4,915,505 to Arribau et al. attempted to overcome the geyser problem by connecting the suction pump and the mix/discharge centrifugal system to a common driveline. However, such a design brings back the problems associated with the conventional programmable optimum density (POD) mix/discharge centrifugal systems described in U.S. Pat. No. 4,453,829 to Althouse, III, U.S. Pat. No. 4,614,435 to McIntire, and U.S. Pat. No. 4,671,665 to McIntire, where, if any of the suction connections and/or hoses leaked air, then the suction side of such a programmable optimum density (POD) mix/discharge centrifugal system would lose prime and the programmable optimum density (POD) mix/discharge centrifugal system would pack off with proppant and quit pumping.